Even if you do all the common hot rod tricks to an 8 valve, ie: cam, headwork, high comp pistons, etc, you'll probably only end up with an engine that has power = to a stock, well tuned 16 valve. The big advantage to the 8 valve is bottom end torque. It's not that it has more than the 16v, it just comes in much sooner, thus makes it easier to drive off road. That being said, if you could find high compression pistons, use a good header, which I think you already have, you'll probably see some noticeable power gains, but you'll have to run premium. Any real power gains are going to come from raising the compression and stuffing more air/fuel mixture into it, which means modifying the throttle body and injector, or going to a stand alone fuel controller such as a Mega Squirt. By that time you might as well have swapped in another motor.